Cable modems transmit and receive digital data over cable television (CATV) networks. These networks were originally installed to transmit television broadcasts. Recently, many CATV networks have been upgraded to provide two-way data communication. Adding cable modems to existing television broadcasting networks makes those networks function like local area computer networks. The original television sets used to receive broadcasts can now be augmented with terminals and modems able to receive and transmit data.
Converting broadcast CATV networks to cable modem networks requires configuring the network for bi-directional communications. Two-way amplification is added to the system so that terminal signals may be transmitted back to the head-end. In addition, scarce bandwidth is allocated for both forward transmissions from the head-end to the terminals, as well as for reverse transmissions from the terminals back to the head-end.
Most CATV networks are hybrid fiber-coax (HFC) networks. The signals run in fiber optical cables from the head-end center to locations near the subscriber. At that point, the signal is converted to coaxial radiofrequency cables, that run to the subscriber terminals. In the bi-directional data network, a cable modem is installed in every user terminal to demodulate and recover received digital transmissions from the head-end (downstream), and to modulate and transmit digital transmissions back to the head-end (upstream). Similarly a cable modem is installed in the head-end to demodulate and recover received digital transmissions from the terminals, and to modulate and transmit digital transmissions back to the terminals.
A typical cable network configuration includes a head-end with a cable modem termination system (CMTS) transmitting data downstream to a terminal network. Approximately two thousand modems can be connected to the CMTS in a tree network configuration per allocated TV channel. If more cable modems are required, the number of TV channels is increased by adding more channels to the CMTS. The physical extent of the network tends to be tens of kilometers or more. This transmission range requires amplification stages, with signal strength varying substantially from terminal to terminal. In the tree configuration, modems communicate directly with the CMTS only. Modem-to-modem communication is done through the CMTS.
Typical cable network signal parameters are summarized in the following table:
DownstreamUpstreamData rate27-56 Mbit/s3 Mbit/sModulation64/256-QAMQPSK/16-QAMBandwidth  6-8 MHz  2 MHzCarrier frequency50-850 MHz5-50 MHzThe downstream channels carry an aggregate of 27 or 56 MBits/s using 64-quadrature amplitude modulated (QAM) or 256 QAM signals, respectively. The total bandwidth allocated to each downstream data transmission channel is 6-8 MHz. The downstream channels are generally placed between 50 and 850 MHz. The upstream channels carry an aggregate of 3 MBits/s using QPSK or 16-QAM. The total bandwidth allocated to each upstream data transmission is 2 MHz. The upstream channels are generally placed between 5 and 50 MHz.
Cable modems are generally connected to computers or special set-top boxes. Computer connection is usually accomplished through an external connector, an internal PCI bus card, or a wireless adapter. Each cable modem in the network transmits bursts in time slots, with each slot marked as reserved, contention, or ranging. Each upstream channel is divided into mini-slots, nominally 16 bytes longs, but can be set up to 128 bytes long by the CMTS. Upstream bandwidth is 200, 400, 800, 1600 or 3200 kHz. A reserved slot is a time slot that is reserved to a particular cable modem, ensuring that no other cable modem transmits during that time. The CMTS allocates the time slots to the various cable modems through a vendor-specific bandwidth allocation algorithm. Reserved slots are normally used for longer data transmissions.
Time slots marked as contention slots are available for transmission by any cable modem. In current systems if two cable modems transmit during the same time slot, the packets collide and the data is lost. The CMTS will signal that no data was received. The cable modems will re-transmit later at other randomly generated times. Contention slots are normally used for very short data transmissions, especially for requests for a number of reserved slots to transmit more data.
The physical distance between the CMTS and the network cable modems causes the time delay to vary substantially. Generally, the delays are on the order of milliseconds. To compensate, cable modems employ a ranging protocol to compensate the clock of the individual cable modem for the delay so that all received transmissions are aligned to the correct frame boundary as they arrive at the CMTS. In U.S. systems, a number of consecutive time-slots are periodically allocated for ranging. In one example, the cable modem is commanded to transmit in the second time-slot. Upon reception, the CMTS transmits a correction value to the cable modem (e.g., a small positive or negative correction value) for its local clock. There is at least one empty time slot before and after the ranging burst to ensure that the ranging burst does not collide with other traffic. The physical distance between the CMTS and the network cable modem also causes varying levels of transmitted signal attenuation among the different modems in the network. The variation in attenuation from the cable modem to the CMTS can vary more than 15 dB. The other purpose of the ranging slot is to align the received amplitudes from all cable modems in the network. For cost-effective demodulation in the CMTS, amplitude alignment is essential for detecting collisions and for minimizing the bit error rate.
In operation, one downstream channel is normally paired with a number of upstream channels to achieve the balance in data bandwidths required. Since the downstream data are received by all cable modems, the total bandwidth is shared between all active modems on the system. Each modem filters out the data it needs from the stream of data. When many modems share the same channel, the effective throughput is lower by a factor of 100 or even 1000. A scheme for multiplexing multiple packets on the same channel would therefore be highly desirable. It would further be desirable to enable the CMTS to receive and recover interfering packets without forcing re-transmission at later times.
What is needed, therefore, are techniques for multiplexing multiple packets on the same channel, and to enable the CMTS to receive and recover interfering packets without forcing re-transmission at later times.